When you connect to your Thundra account, all the applications and application instances of the currently selected organization will be listed in the main Thundra tool window.
In a typical development environment, there might be multiple number of applications and instances of these applications. If you want to work on a subset of the application instances, you can filter desired application instances by clicking the corresponding tags (application names, versions and stages). Only matching application instances will stay visible, and non-matching ones will be hidden after selection. An example selection of instances can be shown below: